Since its discovery three decades ago, sterol carrier protein-2 (SCP-2) has remained a fascinating protein whose physiological function in lipid metabolism remains an enigma. Its multiple proposed functions arise from its complex gene structure, post-translational processing, intracellular localization, and ligand specificity. The SCP-2 gene has two initiation sites coding for proteins that share a common 13 kDa SCP-2 C-terminus: (1) One site codes for 58 kDa SCP-x which is partially post-translationally cleaved to 13 kDa SCP-2 and a 45 kDa protein. (2) A second site codes for 15 kDa pro-SCP-2 which is completely post-translationally cleaved to 13 kDa SCP-2. Very little is yet known regarding how the relative proportions of the two transcripts are regulated. Although all three proteins contain a C-terminal SKL peroxisomal targeting sequence, it is unclear why all three proteins are not exclusively localized in peroxisomes. However, the recent demonstration that the SCP-2 N-terminal presequence in pro-SCP-2 dramatically modulated the intracellular targeting coded by the C-terminal peroxisomal targeting sequence may account for the observation that as much as half of total SCP-2 is localized outside the peroxisome. The tertiary and secondary structure of the 13 kDa SCP-2, but not that of 15 kDa pro-SCP-2 and 58 kDa SCP-x, are now resolved. Increasing evidence suggests that the 58 kDa SCP-x and 45 kDa proteins are peroxisomal 3-ketoacyl-CoA-thiolases involved in the oxidation of branched chain fatty acids. Since 15 kDa pro-SCP-2 is post-translationally completely cleaved to 13 kDa SCP-2, relatively little attention has been focused on this protein. Finally, although the 13 kDa SCP-2 is the most studied of these proteins, because it exhibits diversity of its ligand partners (fatty acids, fatty acyl CoAs, cholesterol, phospholipids), new potential physiological function(s) are still being proposed and questions regarding potential compensation by other proteins with overlapping specificity are only beginning to be resolved.

中文翻译：

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固醇载体蛋白2的基因结构，细胞内定位和功能作用。

自三十年前被发现以来，固醇载体蛋白2（SCP-2）一直是一种令人着迷的蛋白，其在脂质代谢中的生理功能仍然是一个谜。它的多种拟议功能来自其复杂的基因结构，翻译后加工，细胞内定位和配体特异性。SCP-2基因有两个起始位点，这些起始位点编码共享一个13 kDa SCP-2 C末端的蛋白质：（1）一个位点编码58 kDa SCP-x，该位点在翻译后被部分切割成13 kDa SCP-2。和45 kDa的蛋白质。（2）第二个位点编码15 kDa pro-SCP-2，该位点在翻译后完全切割成13 kDa SCP-2。关于如何调节两个转录本的相对比例还知之甚少。尽管所有三种蛋白质均包含C端SKL过氧化物酶体靶向序列，尚不清楚为什么所有这三种蛋白质并非仅局限于过氧化物酶体。然而，最近的证据表明，前SCP-2中的SCP-2 N端序列显着调节了由C端过氧化物酶体靶向序列编码的细胞内靶向，这可能解释了以下观察结果：总SCP-2的一半是定位在过氧化物酶体之外。现在已解决了13 kDa SCP-2的三级和二级结构，但15 kDa pro-SCP-2和58 kDa SCP-x的三级和二级结构已解决。越来越多的证据表明，58 kDa SCP-x和45 kDa蛋白是与支链脂肪酸氧化有关的过氧化物酶体3-酮酰基-CoA-硫代酶。由于15 kDa pro-SCP-2在翻译后完全裂解为13 kDa SCP-2，因此对该蛋白的关注相对较少。最后，